The purpose of this project is to characterize the in vitro cell-mediated immunity in young and aging mice of various genetic strains. The relationship between the immunodeficiency in aging and the regulation, in the form of suppression of the immune system, is investigated. The topics of present interest are: (1) the regulatory mechanisms of the in vitro development of cytotoxic lymphocytes by suppressor cells, (2) the characterization of suppressor cells, and (3) the characterization of the immunodeficiency in aging and its relationship with suppressor cells.